Make sure chainsaw drops appear above ground

A fix for https://github.com/minetest-technic/technic/issues/137
Chainsaw drops are forced to pop above ground. Also, as asl suggested,
they must not end up too high on a ledge or a pillar.

This also cleans up the code style of chainsaw.lua.
Clean up and improve chainsaw
Fix the chainsaw's protection check
Buff chest sizes

Make the inventory sizes of all the technic chests larger, to provide
a more appreciable benefit from upgrading chests, more in keeping with
the resources spent on the upgrades.

Currently the game engine doesn't handle large forms well.  The size
of an inventory slot is fixed relative to the screen height, and a
form that exceeds either screen dimension will extend off the screen,
making parts of it inaccessible.  The tallest a form can get and remain
usable is 13 slots, and even that slightly overspills the screen height.
The maximum usable width depends on the screen aspect ratio.  For the
narrowest common ratio, 5:4, the widest a form can usably get is 15 slots,
again slightly overspilling.  Combined with the layout of the chest forms,
this implies a maximum practical chest inventory size of 15x7 (= 105),
slightly smaller than the largest Minecraft chests.

To provide roughly equal size steps in the five upgrades from wooden
chest (staying at 8x4 = 32) to mithril chest, the steps need to be of
about 15 slots instead of the former 4.  The new sizes are:

    wooden   8x4  32
    iron     9x5  45
    copper  12x5  60
    silver  12x6  72
    gold    15x6  90
    mithril 15x7 105

To make upgrading from the old chest sizes to the new sizes more
convenient, the inventory size is now set not only upon chest construction
but also when accepting form input, at the same time as rewriting
the formspec.  So after upgrading the technic mod, viewing an existing
chest upgrades it to the new size.  The first time a pre-existing chest
is viewed its form will have the old dimensions, looking broken due
to the inventory background image now having the new number of slots.
The second time it is viewed the form will have the new dimensions,
and the full new number of slots will be usable.
Reduce mithril chest to gold chest size

Due to concerns about form appearance, reduce mithril chest size to avoid
its form exceeding screen height.  This reduces it to the same size as
the gold chest.  Having two chest types of the same size looks silly now,
but will be reasonable as soon as mithril chests get their long-planned
special features.
Add registration code for extractor
Add MV extractor
Refactor a lot of code: make only one registration function for grinder, extractor and electric furnace (compressor will follow soon)
Add MV version of compressor too, remove infinite sand-producing compressor recipe.
Better forcefield control dialog

The size configuration is no longer cleared when exiting the dialog with
<esc>.  The enable/disable toggle button now indicates the current state.
The name of the toggle button now varies according to state, so that
pressing the button multiple times in one state (which can arise due
to lag making the user unsure about whether the first press registered)
only makes the state change that the user requested, rather than toggling
repeatedly.
Fix machine description
Make MV extractor/compressor work
Change the recipe code to be able to handle recipes with more than one input, and use it for the alloy furnace.
Reorganize the init.lua files.
Remove most tube crafting recipes
Add missing entry to template
Add snippet to check if function exists.
Fix brass alloying recipe

Commit d55ecc39f954b33c17ae9a1da4aeff6382fcb790 introduced a bogus
reference to "technic:copper_ingot".
Fix craft guide for alloy cooking et al

A typo in commit d55ecc39f954b33c17ae9a1da4aeff6382fcb790 made recipes
for alloy cooking, compressing, and all other craft types sharing that
machine code, to be shown with three ingredient slots instead of the
correct one or two.
Fix fuel-fired alloy furnace

Commit ee0765804c0a21deeb2f33c22ac1a36cb0db5f43 broke the fuel-fired alloy
furnace, by removing the definition of its formspec that it requires to
set up the form upon construction.
Handheld prospecting tool

The new tool will say whether a target block type is present in a
specified region, to allow for more targeted digging.  It is deliberately
quite weak, with several limitations: only stores enough charge for a
small number of shots; target can only be set by pointing at an example
node; range is limited; accuracy is less than 100%.  Some of these
limitations should probably be ameliorated, but not entirely eliminated,
in the future when we have a better idea of game balance.

The inventory image is only a placeholder.